Ah.. Ok, if he's playing principal bass in an orchestra, there's lots of things he needs to learn.
First among them is, if the section is having intonation problems, he should minimise his vibrato to give them something to lock in to, and he should tell his desk partner to do the same. What happens is the section basses try to phase-lock with each other, and the players literally feel the shared vibration of the whole section's basses, not just there own. But if NOBODY has the center of the note, that doesn't work... and it's the first desk's job to make that possible. Inside first actually has more leverage than the principal because of where they're sitting. Whereas, the instruction for the rest of the section is 'if you're not sure of the intonation, use some vibrato, and you should get a sensation sort of like like a notch in your fingerboard where the note is'.
There's a lot more than that, but that's the part that touches on vibrato...